This invention relates to a casting mold having a novel gating pipe system.
As is well known, a casting mold is assembled with a sand mold, a metal mold or the like, and most molds comprise a gating system. The gating system adjacent to a pouring basin can be generally divided into a sprue, a sprue base, a runner and a gate. The sprue is so designed that molten metal corresponding to the weight of a casting is passed through within a predetermined time. The sprue base, runner and gate are so designed that the energy loss due to the fluid resistance of the molten metal is minimized or the molten metal flows through rapidly and gently. By the term "gating pipe system" used herein is meant a system composed of all or some members of the sprue, sprue base, runner and gate.
In general, the gating system for pouring small castings has been molded with sand and coated with a mold coating material to protect the sand surface and ensure its smoothness. The gating system for medium or large castings has been assembled by joining china clay or chamotte pipes 200 to 600 mm in length.
In these conventional gating systems, however, the following problems arise. In view of the quality of castings and problems upon pouring, the wash of sand at a sprue wall, a sprue base and the like or the pipe-melting of china clay or chamotte sprue pipes as shown in FIGS. 6a and 6b is apt to take place, which results in defective castings in the form of sand inclusion and slag inclusion.
As one of the ways to overcome such defective phenomena, castings with machining allowance for receiving the above mentioned inclusions can be produced and then machined to remove the allowance portions. This method, however, results in a decrease in the product yield and a higher cost of the products due to additional man-hours. Moreover, when the extent of wash of sand and pipe-melting is great, leakage of the molten metal and/or clogging of the runner takes place, whereby the pouring operation becomes impossible.
From the viewpoint of molding operations and molding man-hours, even when it is desired to install the runner and gate at ideal positions according to a mold design, such positions are restricted because the china clay or chamotte sprue pipes must be joined and are subject to the lengths of pipes and the shapes of joints. Moreover, in order to fix these pipes and to prevent breakage due to the collision of the molten metal and thermal impact upon pouring or due to the static pressure of the molten metal, a metal frame of cast iron is generally used and the space between the pipes and metal frames is filled with sand containing a binder which is then firmly hardened by ramming. In the course of the ramming operation, shift, breakage and the like of the pipes at the joint portions take place to lower the operation efficiency and also to cause leakage of the molten metal. Thus, in order to prevent shift and breakage and decrease the molding man-hours, development of a single and longer gating system has been urgently needed. At present, such a single gating system of only about 600 mm or less has been provided, because of various problems such as the difficulty in the production, breakage in transportation and high cost.
When sand having a large degree of freedom is used, great skill and a great number of man-hour for the molding are required, and also the risk of the above described wash of sand and sand inclusion is increased. To solve these problems, the china clay or chamotte sprue pipes have come into use, which in turn have given rise to new problems as described above.
We have carried out extensive research for providing novel casting molds to solve the prior art problems, as summarized below.
First, we conducted pouring experiments, using a steel pipe sprue as disclosed in Japanese Utility Model Laid-Open Publication No. 67007/1973 and Utility Model Publication No. 21205/1974 instead of conventional china clay pipes, for the purpose of solving the problems of the molding operations and man-hours. The steel pipe used was a carbon steel pipe (Japanese Industrial Standards Designation STK30) 50A (outer diameter 60.5 mm, wall thickness 2.3 mm), and 800 kgs of molten ductile cast iron was used. In FIG. 2 of said Utility Model Laid-Open Publication No. 67007/73, it is stated that the wall thickness of steel pipe is 3.0 mm for 200 kgs of molten cast steel, the temperature of which is estimated to be about 200.degree. C. higher than the temperature for pouring molten ductile cast iron (1,340.degree. C.) used in the present experiment. Thus, we presumed that a steel pipe can be used as the sprue.
In the present experiment, the sprue portion was dismembered after casting and cooling, and inspected. In spite of a pouring time of only 35 seconds, the sprue steel pipe had been melted to a degree such that its original shape was not retained at all, and a large degree of metal penetration was observed in the sand around the sprue. Thus, it has been found that a steel pipe is not suitable at least as a sprue.
This is considered to be due to the following reasons. Because the carbon concentration in ductile cast iron is as high as about 15 times that of the steel pipe, the carbon is diffused from the molten metal into the sprue steel pipe material, that is, the steel pipe material is carburized. As a result, the melting point of the steel pipe material is lowered. It is considered that the steel pipe underwent rapid pipe-melting because of the synergistic actions of the carburizing phenomenon and the erosive action by the molten metal stream.
Then, a refractory such as Al.sub.2 O.sub.3 powder was applied as a coating on the inner surface of the steel pipe for the purpose of preventing the carburizing phenomenon, and the above described experiment was repeated.
This coating treatment was conducted by using a commercial mold coating material (trade names Ceramol 55 and Okamold) according to the methods described in Japanese Patent Publication Nos. 7911/59 and 8808/59 to produce a coat thickness of about 0.2 to about 1.0 mm. In all of these cases, pipe-melting was observed. This is considered to be due to the reasons that the carburizing phenomenon could not be prevented because the coated steel pipe was subject to thermal impact upon pouring of the molten metal, a remarkable difference in the thermal expansions between the refractory-coated layer and the steel pipe body, and erosive action by the molten metal stream, and the refractory-coated layer was peeled off to expose the steel pipe surface. In this connection, sprues and the like made of refractory-coated steel pipes which can withstand the above described conditions practically without peeling off and pipe-melting have not yet been developed as commercial products.
On the other hand, as a method for improving the antioxidation property at high temperatures of steel materials, a treatment method of coating the surface thereof with diffused aluminum has been known. The Al-diffusion coating treatment method comprises, for example, embedding a steel material to be treated such as steel pipes in an aluminum cementation agent containing a mixture of Al powder with Al.sub.2 O.sub.3 powder or other powder materials and an accelerator such as ammonium chloride which has been packed in an unsealed metal vessel such as an iron chamber, and then heating these materials to a temperature of 850.degree. to 1,050.degree. C. for 10 to 25 hours.
By such a treatment, the diffusion cementation of aluminum into the steel material takes place and an Al-coated layer consisting essentially of an Fe-Al alloy layer is formed on the surface layer of the steel material. When the resulting coated steel material is subjected to secondary heating in a high-temperature oxidizing atmosphere (for example, when the diffusion-coated steel material is heated upon use in contact with molten metal, or is further exposed to an oxidizing atmosphere at 800.degree. to 1,000.degree. C. for 10 to 120 minutes), an aluminum-coated layer consisting of a dense firm Al.sub.2 O.sub.3 film formed on the outer surface and an Fe-Al alloy layer adjoining thereto is obtained. In the secondary thermal treatment, aluminum in the Fe-Al alloy layer is selectively oxidized rather than iron therein to form the Al.sub.2 O.sub.3 layer. The thin dense Al.sub.2 O.sub.3 film formed on the outer surface serves to inhibit invasion of oxygen atoms and prevent oxidation of the steel material.
After fundamental experiments, we have found that, instead of the conventional china clay or chamotte sprue pipes, the above described Al-diffusion coated steel material or the coated and further secondary heat-treated steel material is an optimum material for the sprue pipe system, which eliminates the wash of sand as well as the pipe-melting in the course of pouring of molten metal, and an effective casting mold can be provided by utilizing a steel material thus-treated.